A transformer has become an essential electronic component for voltage regulation into required voltages for various kinds of electric appliances.
Since the leakage inductance of the transformer has an influence on the electric conversion efficiency of a power converter, it is very important to control leakage inductance.
In the power supply system of the new-generation electric products such as LCD televisions, leakage inductance transformers (e.g. LLC transformers) prevail. Generally, the current generated from the power supply system will pass through a LC resonant circuit composed of an inductor L and a capacitor C, wherein the inductor L is inherent in the primary winding coil of the transformer. At the same time, the current with a near half-sine waveform will pass through a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) switch. When the current is zero, the power MOSFET switch is conducted. After a half-sine wave is past and the current returns zero, the switch is shut off. As known, this soft switch of the resonant circuit may reduce damage possibility of the switch, minimize noise and enhance performance.
FIG. 1 is a schematic exploded view of a conventional leakage inductance transformer. The transformer 1 principally comprises a bobbin 11, an upper covering member 12, a magnetic core assembly 13, and a lower covering member 14. A primary winding coil 111 and a secondary winding coil 112 are wound on the bobbin 11. The output terminals 113, 114 of the primary and the secondary winding coils 111, 112 are directly wound and welded on pins 115, which are perpendicularly extended from the bottom of the bobbin 11. The lower covering member 14 is mounted at the bottom of the bobbin 11. The top portion of the bobbin 11 is sheltered by the upper covering member 12. The magnetic core assembly 13 includes two magnetic cores. The middle legs 131 of these two magnetic cores are embedded into a channel 116 of the bobbin 11. The lateral legs 132 of the magnetic core assembly 13 are contacted with each other to enclose the bobbin 11.
As known, the distance between the middle legs 131 of these two magnetic cores of the magnetic core assembly 13 is possibly altered if the transformer 1 is subject to an external force or other actions. Under this circumstance, it is difficult to precisely control the leakage inductance. In addition, since the distances between the lateral legs 132 of the magnetic core assembly 13 and the primary winding coil 111 or the secondary winding coil 112 are very short after the magnetic core assembly 13 is combined with the bobbin 11, the upper covering member 12 is also used to increase the creepage distance between the magnetic core assembly 13, the primary winding coil 111 and the secondary winding coil 112 so as to increase the electric safety. Moreover, a slab element 121 of the upper covering member 12 and a rib 141 of the lower covering member 14 are also used to separate the primary winding coil 111 from the secondary winding coil 112 and thus increase the electric safety distance therebetween.
In other words, the upper covering member 12 and the lower covering member 14 are necessary for increasing the electric safety of the conventional transformer 1. The conventional transformer 1, however, still has some drawbacks. For example, since the conventional transformer 1 has so many components, the process of assembling the transformer 1 is complicated. The upper covering member 12 and the lower covering member 14 also increase the height of the transformer 1, which causes the transformer 1 difficult to conform to the thin tendency. Furthermore, since the output terminals 113, 114 of the primary winding coil 111 and the secondary winding coil 112 are directly wound and welded on the pins 115, a particular length of the wound pin 115 should be reserved. As a consequence, the height of the transformer 1 is also increased. During the winding and welding processes, the integrity of pins 115 also might be adversely affected, and thus the structure strength of the transformer 1 mounted on the circuit board through the pins 115 and even the electrical connection thereof are deteriorated.
Therefore, there is a need of providing an improved structure of a transformer so as to obviate the drawbacks encountered from the prior art.